Abstract: To enhance the stability and bioavailability of astaxanthin (AST), soy protein isolates (SPI) were used as the raw material to prepare SPI/chitosan (CS) composite nanoparticles for the encapsulation of AST. The study investigated the effect of CS on the encapsulation efficiency of AST. Ca²⁺ were then introduced as stabilizer to investigate their effect on the stability of the SPI/CS/AST nanodispersion and to elucidate the underlying mechanism. The results demonstrated that at a CS concentration of 0.6 mg/mL, the encapsulation efficiency of AST reached 59.57%, representing an 11% increase compared to samples without CS. Under this CS concentration, when Ca²⁺ was added at 0.2 mg/mL, the AST encapsulation efficiency further rose to 73.55%, a 20% improvement over samples without Ca²⁺. Moreover, at a storage temperature of 4 ℃, the retention rate of AST increased by 14% with Ca²⁺ addition. Under ultraviolet irradiation, 0.2 mg/mL Ca²⁺ provided the most effective protection for AST within the nanodispersion, enhancing retention by 23% compared to those without Ca²⁺. Furthermore, the retention rates of encapsulated AST in simulated gastric and intestinal fluids reached 78.35% and 67.18%, respectively. Ultraviolet and fluorescence spectroscopy analysis revealed that Ca²⁺ enhanced stability by influencing hydrogen bonds and hydrophobic interactions in the nanodispersion through ionic bridging, electrostatic neutralization, and conformational regulation. In summary, this study successfully established a highly stable AST nano-delivery system through the synergistic effect of SPI/CS composite wall materials and Ca²⁺, providing a theoretical basis for the application of AST in the development of functional foods.